Heavy duty trucks transport over 85% of the total value of goods and thus play an essential role in the United States (U.S.) economy. This mode of transporting goods occurs globally with continued increase over the last 30 years; thus requiring intense demand for fossil fuel consumption to support the trucking industry. In recent times (e.g., 2011 to 2015), yearly sales of class 8 (heavy duty) trucks in the U.S. have increased from ˜171,000 to ˜250,000/year. For ground fleet vehicles powered by fossil fuels, fluctuations in fuel prices and usage determine efficacy of this mode of transport. Unpredictability in fuel price coupled with global efforts in reducing greenhouse emissions combined with limited implementation of biofuels and renewable energy resources has driven the demand for achieving significantly enhanced fuel efficiency in ground fleet vehicles. An energy audit of class 8 trucks indicates that 60% of all dissipative energy losses occur in the engine, with aerodynamic losses accounting for 21%, rolling resistance contributing 13%, and drive-train and auxiliary losses adding another 6%. Out of these four mechanisms contributing to dissipative losses, this application focuses on adding bio-inspired retrofits to class 8 trucks for reducing aerodynamic losses.
A tractor-trailer moving at highway speeds of 25-31 m/s (55-70 miles per hour, mph) involves the interaction of a fluid (air) with a structure (tractor-trailer) resulting in drag force. Simply put, drag force is the resistance an object must overcome while moving through a fluid. As the air goes over the tractor-trailer, the flow profile deviates significantly from free-stream or the velocity far-away from the tractor-trailer. At the tractor front, the incoming air-stream stagnates, leading to an enhanced pressure called stagnation pressure, which is the sum of dynamic and free-stream pressure. Tires present in both the tractor and the trailer, in addition to gap in-between the tractor-trailer, act as a region of flow separation causing local flow reversal in these areas adding drag. Trailer underskirts have been implemented to minimize generation of flow eddies and thereby reduce the drag energy losses due to interacting vortices.
The flow region behind the tractor-trailer is known as the wake. As the airflow approaches the wake, a pressure drop below the ambient pressure is experienced in order to bend the flow just behind the wake, adding additional drag to the moving vehicle. Moreover, flow separation causes significant pressure drop in the wake, preventing pressure recovery and contributing to aerodynamic drag. The total aerodynamic drag for a truck is therefore, a summation of resistance due to skin friction and pressure drag (drag force, FD), and is geometry dependent. In particular, various design considerations for trucks consider design of the front vehicle facade along with the front spoiler, the angle of inclination of the rear window, and the geometry of the vehicle rear. At highway speeds, in comparison to skin friction, pressure drag contributes almost 90% of the total aerodynamic drag. In the context of fuel efficiency, previous work has shown that an estimated 2-3% in fuel savings can be achieved by reducing the overall aerodynamic losses by 5%.
Evaluating the impact of 5% reduction in overall aerodynamic losses to generate approximately 3% in fuel savings requires consideration of the fuel economies for large ground fleet vehicles. Fuel economy of class 8 trucks at 29 m/s (65 mph), depending on their weight range (10,000 to 36,000 kg) is estimated to be between 2.5-3.4 km/l (6-8 miles per gallon (mpg)). Therefore, average invested fuel costs, for a class 8 truck based on a 112,000 km (70,000-mile) annual mileage is $22,000-$30,000, based on $2.6 per gallon of diesel. Given that more than 2.5 million heavy duty trucks are registered in the United States, a 3% enhancement in fuel efficiency (raising consumptive fuel mileage ratio to 6.2-8.3 mpg) via 5% reduction in aerodynamic drag can result in net savings of ˜$1.6-$2.2 billion annually in the heavy duty truck transportation sector.
Furthermore, the National Highway Traffic Safety Administration (NHTSA) has set new emission rules to achieve a 17% reduction in greenhouse gas emissions for heavy duty-vehicles by 2018, furthering the need for sustained and substantial efforts to enhance fuel efficiency with aerodynamic drag reduction, providing an impactful domain to target. Prior efforts to reduce drag have led to two classes of aerodynamic drag reduction devices: first- and second-generation aerodynamic drag reduction devices. The first-generation devices focused on cab-mounted air fairings, side extenders, deflectors and front-end rounding, resulted in relative decrease in drag of a ˜15-31%, in comparison to when no external devices were added. The second-generation devices consisted of base flaps, trailer skirts, and tractor-trailer gap sealers, further decreasing drag by 3-15%. A comprehensive review of previous experimental efforts to minimize drag for full scale and sub-scale efforts was reported previously. Over the past decade or so, reduction in drag coefficient from 0.79 (no devices) to 0.57 was achieved with addition of several first and second-generation devices to full-scale trucks when tested inside a wind tunnel.
The trailer constitutes up to ˜70-80% of total length of a class 8 truck and provides the largest surface area for the vehicle. Despite this large structure, the trailer has seen minimal exploration for technologies towards drag reduction. Accordingly, there is a need for redesigning the trailer towards potential savings in aerodynamic drag.